All-solid-state batteries (thickness, 25 μm) have been introduced, which can be made thinner than lithium polymer batteries by introduction of a thin film process developed with respect to semiconductors (U.S. Pat. No. 5,338,625). In particular, all-solid-state batteries having constituents made thinner by a thin film process are expected to have several times higher energy density than that of the conventional batteries by continuous lamination of the respective constituents, and have attracted much attention.
In the all-solid-state batteries made thinner, however, it is required that an electrode active material is capable of intercalating lithium in the high density and at a high speed in charging/discharging and a solid electrolyte shows high ionic conductivity against lithium ions.
Materials used for the electrode active material may be exemplified by crystalline materials such as LiCoO2, LiMn2O4, LiNiO2, V2O5, MoO2 and TiS2.
On the other hand, solid electrolytes are classified into crystalline solid electrolytes and amorphous solid electrolytes. Li3PO4—Li4SiO4 as one of the crystalline solid electrolytes has the ionic conductivity of 10−6 to 10−5 Scm−1 and has excellent heat resistance, but has a problem that the ionic conductivity thereof has anisotropy and it is therefore difficult to use in batteries. In contrast, amorphous solid electrolytes basically have poor heat resistance, but the ionic conductivity thereof has no anisotropy. In addition, the following sulfur-type solid electrolytes particularly exhibit higher ionic conductivity as compared with oxygen-type solid electrolytes. The amorphous oxygen-type solid electrolytes may include, for example, Li2.9PO3.3N0.36 and amorphous Li3PO4—Li4SiO4, and these have the ionic conductivity of 10−5 to 10−4 Scm−1. As opposed to this, the amorphous sulfur-type solid electrolytes may include, for example, Li2S—SiS2, Li2S—P2S5 and Li2S—B2S3, as well as solid electrolytes obtained by addition of lithium halide such as LiI or lithium oxygen acid salts such as Li3PO4 to these materials, and these electrolytes have the further high ionic conductivity of 10−4 to 10−3 Scm−1.
In preparing a crystal film of an electrode active material, it is an ordinary way to heat the whole of a substrate in order to rearrange constituent atoms to form a crystal lattice. As described above, however, since solid electrolytes with high ionic conductivity have a low heat resistance temperature, the solid electrolyte layer is heated at the same time in the heating process for crystallization of the electrode active material layer so that the temperature of the solid electrolyte layer exceeds the inherent temperature to cause crystallization, resulting in a decrease in ionic conductivity thereof. When an electrode active material layer is formed on the substrate with the solid electrolyte layer previously formed thereon, therefore, it is necessary to supply rearrangement energy for constituent atoms of the electrode active material layer from any part other than the substrate, in order to form the electrode active material layer so that the temperature of the substrate is not raised and the temperature of the solid electrolyte layer formed on the substrate is not raised.
As a special method for energy supply different from the heating of the substrate, Japanese Patent Laid-Open Publication No. Hei 8-287901 discloses the following method. That is, when a lithium-containing crystalline electrode active material layer is prepared by vapor-deposition, there have been proposed, for the purpose of improving the crystallinity of the electrode active material layer and improving adhesion to a substrate, irradiation of the substrate with an ion beam having the energy of 100 eV or higher as the rearrangement energy for atoms, irradiation of the substrate with oxygen together with the ion beam, and irradiation of the substrate with electromagnetic waves such as high-frequency plasmas or ultraviolet ray.
FIG. 3 shows a basic constitutional unit of an all-solid-state battery. The all-solid-state battery with a structure shown in FIG. 3 is formed by the successive lamination, while dry patterning by means of a metal mask or the like, of a first electrode active material layer 52, a solid electrolyte layer 53, a second electrode active material layer 54 and a second current collector 55 on a first current collector 51 also serving as a substrate. What becomes a problem here is a process of forming the first electrode active material layer 52, the solid electrolyte layer 53 and the second electrode active material layer 54. It is to be noted that the present invention is effective for the process of forming these layers.
FIGS. 4 and 5 show the structures further developed from the structure shown in FIG. 3. FIG. 4 shows an all-solid-state battery with a two-stack parallel structure, and FIG. 5 shows an all-solid-state battery with a two-stack serial structure. It should be noted that the present invention is also effective for all-solid-state electrolyte batteries with the further large stack number as well as all-solid-state batteries with a combination of parallel and serial structures.
At present, LiCoO2, LiNiO2, LiMn2O4, LiV2O5 and the like, which are the main electrode active materials used as the materials of lithium ion batteries, require relatively high temperature for crystal formation, whereas the most crystallization of solid electrolytes usually occur around 300 to 350° C. On the contrary, when a combination of an electrode active material having an extremely low melting point, such as Li, and a crystalline solid electrolyte is used, the electrode active material may melt in formation of the solid electrolyte layer. In the battery process for fabricating batteries with the structures shown in FIGS. 3 to 5, it has been necessary to solve the discrepancy caused by the tolerance temperatures of the individual materials.
To solve such a problem, there has conventionally been used an ion irradiation apparatus. The examples of the ion irradiation apparatus may include an ion sputtering apparatus with an accelerating voltage of a hundred kilovolt to several tens of kilovolts, and an ion plantation apparatus with a voltage higher than that; however, when these apparatuses are used and ions with a high energy of 100 eV or higher are irradiated according to the prior art as described, for example, in Japanese Patent Laid-Open Publication Hei 8-287901, the following problems may occur.
The first problem is a decrease in ionic conductivity by the temperature rise of a solid electrolyte layer. The temperature of the surface area of a portion serving as a base material to form a film or a layer drastically rises by an ion collision. When the surface for forming a film thereon (base material surface) is a solid electrode layer, for example, there arises a problem that the ionic conductivity of the surface area of the solid electrolyte layer is impaired and the performance of a battery finally obtained is deteriorated. This is because the ions have a mass close to that of atoms so that the temperature of atoms having collided with the ions and the temperature around these atoms arise to cause partial crystallization of the solid electrolyte layer. A decrease in ionic conductivity is a very important indicator because it restricts the current of the battery, and a decrease in ionic conductivity in the process as described above thus cannot be ignored.
The second problem is that the grade of crystallinity obtained by ion irradiation has a limit. The cause of the limit on the grade of crystallinity is mainly in that the collision of an electrode active material with ions adds energy needed for crystallization and at the same time the collision of the already crystallized electrode active material layer with ions causes disturbance of crystal lattices. In particular, the crystal lattices of the electrode active material layer are portions into which Li ions are inserted and also are passages though which Li ions diffuse, and hence the disturbance of these crystal lattices causes a decrease in capacity of the battery and a decrease in current for charge and discharge.
Further, when an oxide film or a nitride film is formed by an ordinary vacuum film formation process, the ratio of a gas with high vapor pressure, such as oxygen or nitrogen, has a tendency to decrease from the atmosphere. This is mainly because these gases are scattered by a difference in vapor pressure and released outside by a vacuum pump. This phenomenon also causes the disturbance of crystal lattices of the electrode active material layer, resulting in decreases in capacity of the battery, current for charge and discharge, and the like. To solve such a problem, ions such as oxygen ions or nitrogen ions have conventionally been irradiated. This method is already known to be effective as a measure against atom defects; however, from the reasons that the ions used therein are limited to those with low energy and the ion current has an upper limit, there arises a problem that the speed of film formation is remarkably low, i.e., several angstroms per minute or lower.
It is thus an object of the present invention to provide an electrochemical device, having a layer structure causing no deterioration in the characteristics of various materials as described above, as well as a method for preparing the same, which solve the problems on the battery fabrication process as described above.